Screw-cutting

The thread now standardised in British industry is the ISO metric thread, ISO being the International Organisation for Standardisation. Terminology of this thread is shown in Appendix 1. 

The ISO metric thread has a 60° truncated form, i.e. the thread does not come to a sharp point but has a flat crest. The root of the thread also has a small flat. 

The longitudinal travel of the carriage is obtained from the leadscrew through a split nut housed in the apron and operated by a lever on the apron front, Fig. 9.33. By closing the split nut, the drive can be started at any position. 

The position of engagement of the spUt nut on the leadscrew for each cut is important in order that the tool will travel along the same path as the previous cut. To achieve this accuracy of engagement, a thread indicator dial is fitted at the end of the apron, Fig. 9.34. The dial is mounted on a spindle at the opposite end of which is a gear in mesh with the leadscrew. 

These gears are interchangeable, are stored on the spindle, and are selected by referring to a chart on the unit. They are arranged to give a multiple of the pitch required, relative to the 6 mm pitch of the leadscrew. 

The chart shows the gear used for a particular pitch of thread and the numbers on the thread indicator dial at which the split nut may be engaged. To cope with the different diameters of gears, the unit pivots and is locked in position when the gear is in mesh. To avoid unnecessary wear, the unit is pivoted back out of mesh when not in use for screw-cutting. 

Flaving turned the workpiece to the correct diameter, the following procedure should be followed. This procedure is for screw-cutting a right-hand external metric thread on a machine having a metric leadscrew. 

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The corresponding screws cut against one another through a spatial curve. Therefore, the effective clearance orientated to the spatial curve is relevant as it is in view of the selfwiping effect and the product stress. The result is the spatial offset with the aforementioned advantages. ... 

Polyethylene is rather soft, making PE-based composite deck boards easier to nail, screw, cut, and saw. Polyethylene, as well as polypropylene, shows near-zero moisture absorption (typically below 0.02% after 24 h underwater immersion) and... 

The smooth wire nail, so commonly used to-day, readily enters wood with the minimum danger to splitting, but for the same reason it may easily come out of the wood again. This led to the invention of a threaded nail or screw. It seems impossible to discover when screws were first used it cannot have been later than the fourth century for, in the literature of that period an account is given of the chasing of screws by hand. At first they were known as screw-nails . The screw cutting lathe was known to one Jacques Besson in 1548, but Maudsley was the first to make really accurate screws in 1800 to 1810. 

Machines in the most simple of forms utilizing the basic fundamentals of modem machine tools have been existing for many thousands of years. Examples are bow drills and potter s wheels found in ancient Eg3 pt and pole lathes from before the industrial revolution. Modem concepts of a machine tool began to emerge around the sixteenth century when screw-cutting lathes were developed. Various advancements such as metal frames, more accurate pitch control, and more sophisticated power sources all contributed to the steady advancement of machining ease and improved quality (Rolt 1965) (Fig. 1). 

Set the tool relative to the axis of the workpiece, using the screw-cutting gauge. Fig. 9.35. 

Figure 932. Screw-cutting tool for metric thread...

When the end of the workpiece is reached, unwind the cross-sUde to remove the tool from the work and disengage the split nut This is done in one movement At aU times during screw-cutting, one hand should he resting on the cross-sUde hand-wheel, the other on the spUt-nut lever. 

What is the purpose of the thread cutting dial during screw-cutting ... 

Screw cutting. Niobium may be screw-cut using a standard die-cutting head provided that an ample amount of lubricant is used. The use of sufficient lubricant prevents galling on the die, resulting in the tearing of the thread. RoU threading is an alternate, and preferred, method. 

The gearbox, fitted on the lower side of the headstock, provides the range of feeds to the saddle and cross-slide through the feed shaft, and the screw-cutting range through the leadscrew. 

Take great care before engaging automatic feeds and when screw-cutting. 

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